1. Field of the Invention
The present invention relates to a power supply system for driving reactor coolant recirculation pumps in an advanced boiling water nuclear reactor and, in particular, to a power supply system for driving reactor coolant recirculation pumps which has a simple configuration and which is capable of keeping the plant functioning with a high degree of reliability if components thereof should fail.
2. Description of Related Art
In the power-generating plant of an advanced boiling water reactor (hereinafter abbreviated to ABWR), the coolant of the reactor is circulated by a number of reactor coolant recirculation pumps, such as ten pumps (where each is an reactor internal pump, hereinafter abbreviated to RIP). These RIPs use an internal recirculation method to circulate the coolant directly within the reactor of the ABWR.
The amount of coolant circulated by these RIPs is related to the reactivity of the core of the reactor, so this fact can be used to control the amount of coolant that is circulated by the RIPs and thus control the output of the reactor. Any halting of the RIPs by unforeseen causes has an effect on the health of the reactor (coolant) recirculation system (hereinafter abbreviated to RRS), and it could also stop the entire generating plant. Thus the power supply system for driving the RIPs must have a configuration such that the flow rate in the RIPs can be controlled and also the plant can be kept functioning with a high degree of reliability if components of the power system should fail.
The configuration of a prior-art power supply system for driving the reactor coolant recirculation pumps of an advanced boiling water nuclear generating plant is shown in FIG. 35. In this power supply system, an RRS has ten recirculation pumps RIP and each recirculation pump RIP has a static adjustable-frequency power supply device (an adjustable-speed drive, hereinafter abbreviated to ASD). Each static adjustable-frequency power supply device ASD causes changes in the power frequency to control the speed of the corresponding RIP, thus adjusting the flow rate through the core to implement output control of the reactor. An inverter method is generally used for this.
In this prior-art power supply system for driving reactor coolant recirculation pumps, the ten recirculation pumps RIP are divided into two groups of five pumps connected to two normal-operation busbars A and B. The normal-operation busbars A and B are designed to supply power to auxiliary devices of the generating plant, such as the recirculation pumps RIP, and each is connected to a main on-site power line 2 by a house transformer HT. These normal-operation busbars A and B are provided as metal-enclosed switchgear (M/C), and each is designed so that the cables of auxiliary generating equipment such as the recirculation pumps, reactor water feed pumps, etc. can be connected thereto.
With each of the normal-operation busbars A and B, two of the static adjustable-frequency power supply devices ASD are connected directly to the busbar and three of the static adjustable-frequency power supply devices ASD are each connected thereto by an MG set 3, which is usually configured of an electric motor M and a generator G with a flywheel FW attached. Such an MG set 3 causes the generator G with attached flywheel FW to rotate by driving the electric motor M, enabling electrical power to be supplied to the corresponding recirculation pumps RIP.
The main on-site power line 2 is supplied with electrical power by a main generator SG that is rotated by steam from the reactor to generate electricity. The power supplied to the main on-site power line 2 is supplied to the recirculation pumps RIP through the house transformers HT on the one hand, and it is also transmitted to external transmission cables through a transmission transformer MT.
With this prior-art power supply system for driving reactor coolant recirculation pumps in an advanced boiling water nuclear generating plant, the power generated by the main generator SG is supplied to the static adjustable-frequency power supply devices ASD through the house transformers HT, normal-operation busbars A and B, and MG sets 3, etc., and the frequencies of voltages flowing thereto can be controlled by the static adjustable-frequency power supply devices ASD to control the rotational speeds of the corresponding RIPs. This adjusts the flow rate of coolant passing through the core, enabling control over the output of the reactor.
This configuration also makes it possible to prevent three or more recirculation pumps RIP from stopping simultaneously, even if a single failure occurs in one of the MG sets 3 or static adjustable-frequency power supply devices ASD that configure the power supply system for driving reactor coolant recirculation pumps.
However, with this prior-art power supply system for driving reactor coolant recirculation pumps, one static adjustable-frequency power supply device ASD is provided for driving each of the recirculation pumps RIP. Thus the installation configuration is complicated and also expensive. With a prior-art system of this configuration, a failure caused by the stopping of a recirculation pump RIP by the failure of one static adjustable-frequency power supply device ASD goes no further than that one unit and so has little effect, but, on the other hand, there is a large number of static adjustable-frequency power supply devices ASD. For that reason, there has long been a requirement in the art to provide the power supply system for driving reactor coolant recirculation pumps with an installation configuration that is as simple as possible.
Furthermore, if one MG set should fail in the above-described prior-art power supply system for driving reactor coolant recirculation pumps, three recirculation pumps RIP are halted. However, if three or more RIPs are halted simultaneously, it is difficult to absolutely guarantee the flow rate of coolant through the core of the ABWR, which could lead to a drop in the rated output of the nuclear generating plant.
Thus there is a requirement in the art, particularly in the development of the next generation of boiling water reactors, to provide a configuration that ensures that simultaneous failures of RIPs are restricted to no more than two RIPs, a 100% flow rate is guaranteed without any problems in the core even when two RIPs have halted (with at least eight RIPs still operating), and thus the rated output of the nuclear generating plant can be maintained.
An objective of this invention is therefore to provide a power supply system for driving reactor coolant recirculation pumps that has a simple configuration with a reduced number of ASDs in the ABWR.
Another objective of this invention is to provide a power supply system for driving reactor coolant recirculation pumps in which there is an extremely low probability of any number of RIPs halting simultaneously, other than two RIPs, and which makes it possible to maintain the rated output of the plant if two RIPs should halt simultaneously.